How Did Covid Affect Seattle Housing Price

Executive summary

The goal of this time series project is to address the question: ā€œHow did Covid affect the housing price in Seattleā€. The data set is obtained from Zillow, and it contains monthly housing price data. A variety of techniques was used: the data set was first splitted into training and testing set; then the series is differenced to remove trend and seasonality; two model was fitted given the sample ACF/PACF; A series of diagnostic checking tests are performed to the models.

The project then proceeds with the better model, and performs forecasts using the best model. Turns out a heavy tail distribution might be more desirable to fit this data set. However, given the best model in this project, Covid affected Seattleā€™s housing price in a negative way.

All these steps are covered in detail in the main body of the report below:

Introduction

After going to school in Seattle(University of Washington) for 4 years, I found Seattle to have an very unique personality. It has a balanced blend of metropolitan and nature; Itā€™s surrounded by the ocean and mountains; It has traditions while also extremely forward thinking. It is a charming place and here are some of the pictures that I took while I was there.

At the same time, Seattle is groud zero for Covid in the US, as it has seen the first U.S covid case. On the individual scale, everyoneā€™s life has changed since then. On the scale of the world, it has slowed down the global economy. Housing prices have always been an indicator of the state of the economy, since the desire of buying a house is relatable to the vast majority. Therefore, the aim of this project is to perform forecast as if Covid did not happen and study how Covid affected housing prices in Seattle.

The plan is to fit a model given this data set(obtained from Zillow), and perform forecast as if Covid did not happen, then compare the predicted values to the actual values to see how the pandemic changed the housing market in Seattle.

The data set includes average housing prices of cities in the US on a monthly basis. Itā€™s very complete data set and it records the prices from the 90ā€™s until present day. The data is interesting because most people will be looking to buy a house at some point in their lives, and by looking at the past data, it will give people a better idea of how much they can expect to pay for a house.

The techniques include log transformation to the original data set;differencing the time series to remove trend and seasonality; Fit the data set using sarima models by examining the sample ACF/PACF; Diagnostic checking; Forecast on the transformed data and original data; Spectral analysis.

Key results were the best model is a seasonal MA model; The residuals passed most of the diagnostic checking tests except the Shpiro-Wilk test; Residuals also passed spectral analysis tests.

A concise conclusion: even though the seasonal MA model passes most of the tests,a heavy tail distribution might work better; Using the model from this project, Covid affected Seattleā€™s housing price in a negative way.

This project is implemented in R.

Section 1: Initial Analysis

Initial analysis is done in this first section. A plot/histogram of the raw time series is plotted as well as the plot of the the Time series differences at lag 1.

An initial look at the raw time series data shows there is an upward trend(in red).

After differences at lag = 1 to remove the trend. The ts plot shows a significant drop around k = 300. That wouldā€™ve been the result of the Covid 19 pandemic.

Therefore,the data set will leave out the last 50 data points to build the model. From the rest of the data set, the last 12 data points will be used as a validation set

Section 1.1: Transformation

In this section, a box cox plot is generated to check if any transformation is necessary:

By performing the box cox check, 0 is contained in the CI. Therefore, a log transformation will be applied to the data set. The variance of ts decreases by comparing the plot before and after the log transformation.

Also the histogram shows, a log transformation gave a more symmetric histogram.

Section 1.2: Remove Trend/Seasonality

In this section, the ts is differenced to remove trend and seasonality: The ts is first differenced at lag 1 because the original data shows an upward trend. Then by checking the ts plot after differenced at lag 1, it still shows a trend. Therefore, difference at lag 1 once more to remove the trend.

After differenced at lag 1 twice, the ts shows seasonality. Therefore, difference the ts at lag 12 to remove seasonality.

By checking the ts plot after differenced at lag 1 twice and lag 12 once. There is not a trend(red line), and seasonality. The variance is also more stable.

After the procedures mentioned above, the ts is stationary.

ts Differenced at Different Lags ts Variance
ts Difference at Lag 1 0.0001118
ts Difference at Lag 1 Wwice 0.0000284
ts Difference at Lag 1 Twice, Then at Lag 12 0.0000281
ts Difference at lag 1 twice, Then at Lag 12, and at Lag 1 0.0000412

From the variance chart above. Differencing at lag 1 twice and lag 12 once produces the lowest variance. Further differencing actually increases the variance. Therefore, the project will proceed with differencing at lag 1 twice and lag 12 once.

Section 2: Model Identification/Selection

In this section, the ACF/PACF plots are produced to help determine a model:

Section 2.1 Identify/ Fit possible Model #1

Upon examining the ACF, itā€™s outside the CI at log 1 and 12. Since the seasonality is 12. This could indicate Q = 1 and q = 1. Upon examining the PACF, it demonstrate a exponential decay pattern between the years as well as within the year. Therefore, the first candid model is SARIMA(0,2,1)(0,1,1) s =12.

Fit 1 Coefficients
Coef S.E
ma1 -0.3559014 0.0761016
sma1 -0.8811392 0.0544728

Upon examining the CI for both coefficients. 0 is not contained in the CI, therefore, both coefficients are significant.

To write the model in algebric form: \(X_t = (1-B)^2(1-0.3559B)(1-B^{12})(1-0.88B^{12})Z_t\)

Section 2.2 Diagnostic checking of Model #1

Diagnostic Tests for Fit 1
Test P-Value
Shapiro Wilk 0.0000000
Box-Pierce 0.6203464
Ljung-Box 0.5869381
Mcleod-Li 0.8662669

Upon examining the plots, the histogram of the residuals show an approximate normal distribution with a few of outliers. The residual plot shows no trend and stable variance except the first 20 observations. The QQ plot suggests itā€™s a normal distribution. ACF and PACF are all within the CI. There is no unit roots because there is only the MA part, and the coefficients are strictly less than 1.

Upon examining the chart, the residuals did not pass the Shapiro Wilk test. Therefore, reject the null hypothesis that the residual has a Gaussian distribution.

Since the residuals displays an abnormal jump around time 20. The following section is to remove the first 20 obersvations of the residual and perform the diagnostic checking again

Diagnostic Tests for Fit 1 After Removing the First 20 Observations
Test P-Value
Shapiro Wilk 0.0025238
Box-Pierce 0.1425368
Ljung-Box 0.1121810
Mcleod-Li 0.0000155

After removing the first 20 observations, the residual still didnā€™t pass Shapiro Wilk test. At the same time, it fails the Mcleod Li test. The project will proceed with the original residual.

Section 2.3 Identify/ Fit possible Model #2

Upon examining the ACF, itā€™s outside the CI at log 1 and 12. Since the seasonality is 12. This could indicate Q = 1 and q = 1. Upon examining the PACF, the seasonal pattern can be interpreted as P = 3, within the year, p =1 . Therefore, the second model is SARIMA(1,2,1)(2,1,1) s =12.

Fit 2 Coefficients
Coef S.E
ar1 -0.0526922 0.0696598
ma1 -0.3054904 0.4000291
sar1 -0.1284971 0.0741804
sar2 -0.1930561 0.0502129
sma1 -0.7730429 0.0778808

From the estimate of the coefficients, one could construct a CI for the coefficients. The CI for ar1 contains 0, therefore, the model will be adjusted to SARIMA(0,2,1)(2,1,1) s =12.

Coef S.E
ma1 -0.3544822 0.0763583
sar1 -0.1254902 0.1016225
sar2 -0.1907180 0.0905919
sma1 -0.7736512 0.1013464

From the estimate of the coefficients, one could construct a CI for the coefficients. The CI for sar1 contains 0, therefore, the model will be adjusted to SARIMA(0,2,1)(2,1,1) s =12 with the coefficient for sar1 being fixed to be 0.

Coef S.E
ma1 -0.3563616 0.0763440
sar1 0.0000000 0.0000000
sar2 -0.1398785 0.0794084
sma1 -0.8440430 0.0642367

From the estimate of the coefficients, one could construct a CI for the coefficients. The CI for sar2 contains 0, therefore, the model will be adjusted to SARIMA(0,2,1)(0,1,1) s =12 with the coefficient for sar1 being fixed to be 0. This model reduces to the first fit.

Since the 2nd model reduces to the first model, itā€™s not necessary to compare the AICcā€™s

Therefore, this project will proceed with the first fit, and in algebraic form: \((1-B)^2(1-B^{12})X_t = (1-0.3559B)(1-0.88B^{12})Z_t\)

Section 3: Forcast

Forecast will be implemented in this section.

Section 3.1 : Forecast on Transformed Data

This section is dedicated to implement forecasting on the log transformed data: The forecasts are within the CI.

Section 3.2 : Forecast on Original Data

This section is a forecast on the original data:

Now add the original ts data(in black)

The Prediction is done for Jan, 1, 2019. Test CI is very narrow. The actual ts data is touching the lower bound of the CI. This could be a heavy tail distribution. Residuals test from the previous sections also suggest itā€™s head-tail distribution.

Section 4: Examine How Covid Changed Seattleā€™s Housing Prices

The more advanced goal of this project is to study how Covid changed Seattleā€™s Housing Market. A 48 head prediction will put the ts right in the middle of the pandemic

Upon examining the forecast plot. The actual housing prices(black) is much lower than the predictions(purple). Therefore, itā€™s reasonable to suggest that Covid affected Seattleā€™s housing market negatively.

Section 5: Spectral Analysis

This section is dedicated to spectral analysis

Fisher test
P-value
Fisher Test 0.6488167

Upon examining the periodogram for the transformed data, the frequency is around 0.8, which corresponding to a period of 12, which is whatā€™s expected.

From looking at the periodogram of the residuals, there is no dominant frequency, which is desired.

The P value Fisher test is larger than 0.05. Fail to reject the null hypothesis that the residual is Gaussian WN.

Kolmogorov-Smirnov Test for cumulative periodogram of residualsā€”passed. Since the black line is within the blue interval.

Section 6: Conclusion

To recap, this study is aim to study how Covid affected Seattleā€™s housing market. It performed initial analysis on the Seattle housing price time series, and implemented a log transformation on the time series.To remove the trend and seasonality, the log transformed time series is differenced at lag 1 twice and at lag 12 once. Then after fitted different models, it is obvious that the seasonal moving average model is the best model.(\(X_t = (1-B)^2(1-0.3559B)(1-B^{12})(1-0.88B^{12})Z_t\)) The model also passes spectral analysis tests: KS test, Fisher test.However, upon performing diagnostic checking as well as forecasting, it showed that the residuals arenā€™t normally distributed. Therefore, a heavy tail distribution might be more desirable. A heavy tail distribution will widen the CI and as shown in the forecast section, the CI of the current model is too narrow. This could be future study on this topic.

To address the purpose of the study, which is to study how covid affected Seattleā€™s housing prices. The current model is still useful since it passed the majority of the tests, and upon performing a 2 year ahead forecast. It showed Covid had a negative effect on the housing market, since the predicted values is much higher than the actual values.

Overall, the goal of this project is achieved: a various of ts techniques were studies and used on this real-life data set and the model made reasonable predictions.

A Huge Thank you to Prof.Feldman For Teaching and Helping me with this project! and a Huge Thank you to my TA Thiha for helping me with this project!

Section 7: References

All the pictures in this project were taken by me.

This data set is obtained from Zillowā€™s Website: Zillow Website

The Lecture slides helped to build this project: 274 Lecture Slides

Appendix

Section 1: Initial Analysis

house_prices = read.table("houseprices.csv", header = FALSE,
    sep = ",")
dim(house_prices)
class(house_prices)
seattle_df = t(rbind(house_prices[1, 6:331], house_prices[17,
    6:331]))
sea_ts = ts(seattle_df[, 2], start = c(1996, 3), frequency = 12)
sea_ts = as.numeric(sea_ts)
ts.plot(sea_ts)
nt = length(sea_ts)
fit <- lm(sea_ts ~ as.numeric(1:nt))
abline(fit, col = "red")
mean(sea_ts)[1]
abline(h = mean(sea_ts))
title(main = "Raw Time Series Plot")
hist(sea_ts, col = "lightcyan", xlab = "", main = "histogram of raw data")


sea_ts_1 <- diff(sea_ts, lag = 1)
sea_ts_1 = ts(sea_ts_1)
ts.plot(sea_ts_1)
title(main = "Time Series Plot differenced at lag = 1")
hist(sea_ts_1, col = "lightcyan", xlab = "", main = "histogram of data differenced at lag = 1")

Section 1.1:

ts_test = sea_ts[c(268:280)]  # Define Testing set 
ts_train = sea_ts[c(1:268)]  # Define Training set

bcTransform <- boxcox(ts_train ~ as.numeric(1:length(ts_train)))
# Box Cox transformation
lambda = bcTransform$x[which(bcTransform$y == max(bcTransform$y))]

# Perform a log transformation Since the Box Cox
# transformation interval contains 0.
ts_train_log <- log(ts_train)

# Plot ts after log transformation
ts.plot(ts_train)
title(main = "Raw Time Series Plot")
ts.plot(ts_train_log)
title(main = "Time Series Plot after transformation")

# compare histograms after log transformation
hist(ts_train, col = "lightcyan", xlab = "", main = "Histogram Before Log Transformation")
hist(ts_train_log, col = "lightcyan", xlab = "", main = "Histogram After Log Transformation")

Section 1.2: Remove Trend/Seasonality

# The ts plot shows trend Difference at lag 1 to remove
# seasonality
sea_ts_1 <- diff(ts_train_log, lag = 1)
ts.plot(sea_ts_1)
title(main = "Difference at Lag 1")
var(sea_ts_1)

# There is still a trend, Difference at lag 1 again
sea_ts_1_1 <- diff(sea_ts_1, lag = 1)
ts.plot(sea_ts_1_1)
abline(h = mean(sea_ts_1_1), col = "blue")
fit <- lm(sea_ts_1_1 ~ as.numeric(1:length(sea_ts_1_1)))
abline(fit, col = "red")
title(main = "Difference at Lag 1, 1")
var(sea_ts_1_1)


# The ts plot shows seasonality Difference at lag 12 to
# remove seasonality
sea_ts_1_1_12 <- diff(sea_ts_1_1, lag = 12)
ts.plot(sea_ts_1_1_12)
abline(h = mean(sea_ts_1_1_12), col = "blue")
fit <- lm(sea_ts_1_1_12 ~ as.numeric(1:length(sea_ts_1_1_12)))
abline(fit, col = "red")
title(main = "Difference at Lag 1, 1, and 12")
var(sea_ts_1_1_12)

hist(sea_ts_1_1_12, col = "lightcyan", xlab = "", main = "")

sea_ts_1_1_12_1 <- diff(sea_ts_1_1, lag = 1)
var(sea_ts_1_1_12_1)
# Making Variances at different ts differences into a
# Table.
variances <- c(var(sea_ts_1), var(sea_ts_1_1), var(sea_ts_1_1_12),
    var(sea_ts_1_1_12_1))
row_names <- c("ts Difference at Lag 1", "ts Difference at Lag 1 Wwice",
    "ts Difference at Lag 1 Twice, Then at Lag 12", "ts Difference at lag 1 twice, Then at Lag 12, and at Lag 1")

variance_table <- data_frame(row_names, variances)
knitr::kable(variance_table, "pipe", align = c("l", "c"), col.names = c("ts Differenced at Different Lags",
    "ts Variance "))

Section 2: Model Identification/Selection

# Check the acf/ pacfs
acf(sea_ts_1, lag.max = 100, main = "ACF at lag 1")
pacf(sea_ts_1, lag.max = 100, main = "PACF at lag 1")

acf(sea_ts_1_1, lag.max = 100, main = "ACF at lag 1 twice")
pacf(sea_ts_1_1, lag.max = 100, main = "PACF at lag 1 twice")

acf(sea_ts_1_1_12, lag.max = 50, main = "ACF at lag 1 twice and lag 12 once")
pacf(sea_ts_1_1_12, lag.max = 50, main = "PACF at lag 1 twice and lag 12 once")

Section 2.1 Identify/ Fit possible Model #1

Diagnostic Tests for Fit 1
Test P-Value
Shapiro Wilk 0.0000000
Box-Pierce 0.3393664
Ljung-Box 0.3116902
Mcleod-Li 0.6204202 
library("forecast")
fit_1 <- arima(ts_train_log, order = c(0, 2, 1), seasonal = list(order = c(0,
    1, 1), period = 12), method = "ML")

fit_1_coef <- c(fit_1$coef)
fit_1_coef_se <- c(sqrt(fit_1$var.coef)[1], sqrt(fit_1$var.coef)[4])
fit_1_table <- data.frame(fit_1_coef, fit_1_coef_se)
knitr::kable(fit_1_table, "pipe", align = c("l", "c"), col.names = c("Coef",
    "S.E"), caption = "Fit 1 Coefficients")

Section 2.2 Diagnostic checking of Model #1

res_1 <- residuals(fit_1)  # residuals for fit 1. 
hist(res_1, density = 20, breaks = 100, col = "purple", xlab = "",
    prob = TRUE, main = "Residuals Histogram of Fit 1")

m1 <- mean(res_1)
std1 <- sqrt(res_1)

plot.ts(res_1)
title(main = " Residuals of Fit 1 Plot")

# Residual Trend/Mean plot
fitt_1 <- lm(res_1 ~ as.numeric(1:length(res_1)))
abline(fitt_1, col = "pink")
abline(h = mean(res_1), col = "blue")

# QQ plot for the residual of model 1
qqnorm(res_1, main = "Normal Q-Q Plot for Model 1")
qqline(res_1, col = "blue")

# Check ACF/PACF for the residual of model 1
acf(res_1, lag.max = 40, main = "ACF of the Residuals of Fit 1")

pacf(res_1, lag.max = 40, main = "PACF of the Residuals of Fit 1")


# Checking Invertibility of the MA part.
source("plot.roots.R")
plot.roots(NULL, polyroot(c(1, -0.3559)), main = "(A) Roots of MA part, Nonseasonal ")

# Tests
fit_1_SW <- shapiro.test(res_1)

fit_1_BP <- Box.test(res_1, lag = 12, type = c("Box-Pierce"),
    fitdf = 2)
fit_1_LB <- Box.test(res_1, lag = 12, type = c("Ljung-Box"),
    fitdf = 2)
fit_1_ML <- Box.test((res_1)^2, lag = 12, type = c("Ljung-Box"),
    fitdf = 0)

fit_1_diag_results <- c(fit_1_SW$p.value, fit_1_BP$p.value, fit_1_LB$p.value,
    fit_1_ML$p.value)
fit_1_diag_test <- c("Shapiro Wilk", "Box-Pierce", "Ljung-Box",
    "Mcleod-Li")
fit_1_diag_table <- data_frame(fit_1_diag_test, fit_1_diag_results)
knitr::kable(fit_1_diag_table, "pipe", align = c("l", "c"), col.names = c("Test",
    "P-Value"), caption = "Diagnostic Tests for Fit 1")
res_1_2 <- res_1[20:268]
plot.ts(res_1_2)
title(main = " Residuals of Fit 1 Plot After Removing the First 20 Observations")

# Tests

fit_1_2_SW <- shapiro.test(res_1_2)

fit_1_2_BP <- Box.test(res_1_2, lag = 12, type = c("Box-Pierce"),
    fitdf = 2)
fit_1_2_LB <- Box.test(res_1_2, lag = 12, type = c("Ljung-Box"),
    fitdf = 2)
fit_1_2_ML <- Box.test((res_1_2)^2, lag = 12, type = c("Ljung-Box"),
    fitdf = 0)

fit_1_2_diag_results <- c(fit_1_2_SW$p.value, fit_1_2_BP$p.value,
    fit_1_2_LB$p.value, fit_1_2_ML$p.value)
fit_1_diag_test <- c("Shapiro Wilk", "Box-Pierce", "Ljung-Box",
    "Mcleod-Li")
fit_1_2_diag_table <- data_frame(fit_1_diag_test, fit_1_2_diag_results)


knitr::kable(fit_1_2_diag_table, "pipe", align = c("l", "c"),
    col.names = c("Test", "P-Value"), caption = "Diagnostic Tests for Fit 
             1 After Removing the First 20 Observations")

Section 2.3 Identify/ Fit possible Model #2

fit_2 <- arima(ts_train_log, order = c(1, 2, 1), seasonal = list(order = c(2,
    1, 1), period = 12), method = "ML")
fit_2_coef <- c(fit_2$coef)
fit_2_coef_se <- c(sqrt(fit_2$var.coef)[1], sqrt(fit_2$var.coef)[7],
    sqrt(fit_2$var.coef)[13], sqrt(fit_2$var.coef)[19], sqrt(fit_2$var.coef)[20])
fit_2_table <- data.frame(fit_2_coef, fit_2_coef_se)
knitr::kable(fit_2_table, "pipe", align = c("l", "c"), col.names = c("Coef",
    "S.E"), caption = "Fit 2 Coefficients")
fit_2_1 <- arima(ts_train_log, order = c(0, 2, 1), seasonal = list(order = c(2,
    1, 1), period = 12), method = "ML")
fit_2_1_coef <- c(fit_2_1$coef)
fit_2_1_coef_se <- c(sqrt(fit_2_1$var.coef)[1], sqrt(fit_2_1$var.coef)[6],
    sqrt(fit_2_1$var.coef)[11], sqrt(fit_2_1$var.coef)[16])
fit_2_1_table <- data.frame(fit_2_1_coef, fit_2_1_coef_se)
knitr::kable(fit_2_1_table, "pipe", align = c("l", "c"), col.names = c("Coef",
    "S.E"))
fit_2_2 <- arima(ts_train_log, order = c(0, 2, 1), seasonal = list(order = c(2,
    1, 1), period = 12), fixed = c(NA, 0, NA, NA), method = "ML")
fit_2_2_coef <- c(fit_2_2$coef)
fit_2_2_coef_se <- c(sqrt(fit_2_2$var.coef)[1], 0, sqrt(fit_2_2$var.coef)[5],
    sqrt(fit_2_2$var.coef)[9])
fit_2_2_table <- data.frame(fit_2_2_coef, fit_2_2_coef_se)
knitr::kable(fit_2_2_table, "pipe", align = c("l", "c"), col.names = c("Coef",
    "S.E"))

Section 3: Forcast

Section 3.1 : Forecast on Transformed Data

# Forecast on transformed data
pred_transform <- predict(fit_1, n.ahead = 12)
upper_transform = pred_transform$pred + 2 * pred_transform$se
lower_transform = pred_transform$pred - 2 * pred_transform$se
ts.plot(ts_train_log, xlim = c(1, length(ts_train_log) + 12),
    ylim = c(min(ts_train_log), max(upper_transform)))

lines(upper_transform, col = "blue", lty = "dashed")
lines(lower_transform, col = "blue", lty = "dashed")
points((length(ts_train_log) + 1):(length(ts_train_log) + 12),
    pred_transform$pred, col = "red")
title(main = "12 Forecasts on Transformed Data")

Section 3.2 : Forecast on Original Data

# Forecasts on Original data
pred_original <- exp(pred_transform$pred)
U = exp(upper_transform)
L = exp(lower_transform)
ts.plot(ts_train, xlim = c(240, length(ts_train) + 12), ylim = c(250,
    max(U)))
lines(U, col = "green", lty = "dashed")
lines(L, col = "green", lty = "dashed")
points((length(ts_train) + 1):(length(ts_train) + 12), pred_original,
    col = "purple")
title(main = "12 Forecasts on Original Data")
ts.plot(sea_ts, xlim = c(260, length(ts_train) + 12), ylim = c(250,
    max(U)), col = "black")
lines(U, col = "green", lty = "dashed")
lines(L, col = "green", lty = "dashed")
points((length(ts_train) + 1):(length(ts_train) + 12), pred_original,
    col = "purple")

Section 4: Examine How Covid Changed Seattleā€™s Housing Prices

pred_transform_covid <- predict(fit_1, n.ahead = 48)
upper_transform_covid = pred_transform_covid$pred + 2 * pred_transform_covid$se
lower_transform_covid = pred_transform_covid$pred - 2 * pred_transform_covid$se
ts.plot(ts_train_log, xlim = c(200, 326), ylim = c(10, max(upper_transform_covid)))

lines(upper_transform_covid, col = "blue", lty = "dashed")
lines(lower_transform_covid, col = "blue", lty = "dashed")
points((length(ts_train_log) + 1):(length(ts_train_log) + 48),
    pred_transform_covid$pred, col = "red")
title(main = "48 Forecasts on Transformed Data")


pred_original_covid <- exp(pred_transform_covid$pred)
U_covid = exp(upper_transform_covid)
L_covid = exp(lower_transform_covid)
ts.plot(sea_ts)
lines(U_covid, col = "green", lty = "dashed")
lines(L_covid, col = "green", lty = "dashed")
points((length(ts_train) + 1):(length(ts_train) + 48), pred_original_covid,
    col = "purple")
title(main = "48 Forecasts on Original Data")

ts.plot(sea_ts)

Section 5: Spectral Analysis

# Recover frequencies
periodogram(ts_train_log, main = "Periodogram of the Transformed Data")  # period 12
periodogram(res_1, main = "Periodogram of the Residual")  # no dominate frequency 

# Fisher's test for periodicity detection Apply to
# residuals
fisher_results <- fisher.g.test(res_1)  # fail to Reject H0

fisher_table <- data.frame("Fisher Test", fisher_results)

knitr::kable(fisher_table, "pipe", align = c("l", "c"), caption = "Fisher test ",
    col.names = c("", "P-value"))

# KS Test
cpgram(res_1, main = "KS Test")  # pass the test, Gaussian WN

Section 6: Fit coefficents

Coefs for fit 1

fit_1
## 
## Call:
## arima(x = ts_train_log, order = c(0, 2, 1), seasonal = list(order = c(0, 1, 
##     1), period = 12), method = "ML")
## 
## Coefficients:
##           ma1     sma1
##       -0.3559  -0.8811
## s.e.   0.0761   0.0545
## 
## sigma^2 estimated as 1.859e-05:  log likelihood = 1011.11,  aic = -2018.23

Coefs for fit 2

fit_2
## 
## Call:
## arima(x = ts_train_log, order = c(1, 2, 1), seasonal = list(order = c(2, 1, 
##     1), period = 12), method = "ML")
## 
## Coefficients:
##           ar1      ma1     sar1     sar2     sma1
##       -0.0527  -0.3055  -0.1285  -0.1931  -0.7730
## s.e.   0.0697   0.4000   0.0742   0.0502   0.1513
## 
## sigma^2 estimated as 1.834e-05:  log likelihood = 1013.43,  aic = -2016.86
fit_2_1
## 
## Call:
## arima(x = ts_train_log, order = c(0, 2, 1), seasonal = list(order = c(2, 1, 
##     1), period = 12), method = "ML")
## 
## Coefficients:
##           ma1     sar1     sar2     sma1
##       -0.3545  -0.1255  -0.1907  -0.7737
## s.e.   0.0764   0.1016   0.0906   0.1013
## 
## sigma^2 estimated as 1.835e-05:  log likelihood = 1013.41,  aic = -2018.82
fit_2_2
## 
## Call:
## arima(x = ts_train_log, order = c(0, 2, 1), seasonal = list(order = c(2, 1, 
##     1), period = 12), fixed = c(NA, 0, NA, NA), method = "ML")
## 
## Coefficients:
##           ma1  sar1     sar2     sma1
##       -0.3564     0  -0.1399  -0.8440
## s.e.   0.0763     0   0.0794   0.0642
## 
## sigma^2 estimated as 1.839e-05:  log likelihood = 1012.62,  aic = -2019.23